The Miami blue, which had decorated gardens and meadows in coastal Florida from Daytona Beach to St. Petersburg, has retreated to a few islands off Key West. No one knows exactly what accounts for the decline, but the suspects include coastal development, pesticides, climate change and non-native species such as iguanas consuming its host plants.

The North American Butterfly Association and Center for Biological Diversity, an environmental group based in Tucson, Ariz., have both taken legal steps to obtain state and federal protection for the species. The center filed notice of intent to sue the U.S. Fish and Wildlife Service, and on Thursday said the government’s decision gives one of North America’s rarest insects a chance to survive.

“The Miami blue butterfly is on the very brink of extinction, and this finalized protection gives it a real shot at survival and recovery,” said Tierra Curry, a biologist at the Center. “The Endangered Species Act is 99 percent effective at preventing the extinction of the species it covers, so we do have a hope, under the safety net of the act, of stopping the loss of this beautiful butterfly.”

Jaret Daniels, assistant professor of entomology at the University of Florida and assistant curator of the McGuire Center for Lepidoptera & Biodiversity, said priorities should be to study the remaining population to figure out what caused the decline and consider establishing new populations in parts of its old range.

Although its current home in the islands of the Key West National Wildlife Refuge is secure, he said, a single hurricane could wipe it out.

“Once it gets down to such an isolated population, a single local factor could cause it to wink out,” he said. “You could argue that the butterfly doesn’t have a lot of time.”